A few reasons.
1) Some vendors don't want to be undercut in public.
2) Some members will undoubtedly decide to add "their opinion" of the level of discount being offered (i.e. whining and complaining about how vendor XXX is still charging too much and they can get it cheaper for vendor YYY)
3) Some manufacturer's don't like vendors starting price wars on their products in the public. And vendors don't like pissing off their suppliers.
A buyer can still get a good deal but they will have to do some work to get it (versus having it laid out on a silver platter).